The invention relates to an arrangement for measuring the velocity with respect to a surface placed at some distance therefrom. The arrangement comprises a transmitting circuit for directing a wave towards that surface by means of a transmission antenna, a receiver circuit for receiving by means of at least one receiving antenna the wave returned from the surface, and a circuit for processing the returned wave for providing the indication of the velocity.
Such an arrangement is used with great advantage, more specifically in radio navigation for measuring the horizontal speed of an airplane in which the arrangement is installed, the relevant surface then being the ground. Further applications can be found in the textile industry where often, during manufacture, one needs to determine the rate of travel of a piece of fabric.
In Applicant's French Patent Specifications, published under Nos. 2,477,283 (corresponding to U.S. Pat. No. 4,426,647) and 2,502,340, rate measuring arrangements are described. In these known arrangements the receiving circuit includes two receiving antennas which are spaced apart by a certain distance and a correlation circuit for effecting the correlation between the signals pulled-in by the antennas, the maximum of the correlation then defining the speed.